A binary photomask may include glass and chrome features which form a pattern. Light may pass through the clear glass areas and be blocked by the opaque chrome areas. Light that passes through the mask may continue through a lens, which projects an image of the mask pattern onto a wafer. The wafer is coated with a photosensitive film (photoresist), which undergoes a chemical reaction when exposed to light. After exposure, the areas on the photoresist exposed to the light may be removed in a developing process, leaving the unexposed areas as features on the wafer.
The quality of an imaged mask pattern produced with a typical binary mask may be degraded by light from clear areas on the mask diffracting into regions that ideally would be completely dark. The nominally dark region may have light diffracted into it from the adjacent nominally bright regions, thereby affecting the photoresist and quality of the printed pattern. An alternating phase shift mask (APSM) may be used to reduce such diffraction. In the APSM, alternating clear regions (which may be designated as zero (0) and pi (n) regions) may have different step heights which cause the light to be phase-shifted 180° between the two regions. As a consequence, the light diffracted into the nominally dark area from the clear zero region will interfere destructively with the light diffracted from the adjacent pi region. This may improve image contrast on the wafer.
An APSM may be fabricated by patterning all features (apertures) in the chrome layer on the quartz mask substrate in a first (binary) step. After the binary processing, the mask is recoated with resist, and the apertures that are to be quartz etched are then exposed. The open apertures are then etched to a 180° phase depth and “converted” to pi apertures. An isotropic etch step may be used to “hide” the phase edge under the chrome. The requirements for this second patterning step are relatively loose, since the chrome region between the zero and pi apertures on which the resist edges must land, which may be relatively large, e.g., about 200 nm for a 193 nm lithography system.